Pulse width modulated fluidic valve

ABSTRACT

A pulse width modulated fluidic valve includes a cylinder having an elongate bore, a length and first and second ports which extend from outside the cylinder into the bore. A rotatable spool is carried in the bore and movable in a direction of the length of cylinder. The spool has a variable blocking feature which blocks passage of fluid between the first and second ports as a function of angular position relative to the first and second ports and as a function of linear position along the length of the cylinder.

BACKGROUND OF THE INVENTION

The present invention relates to fluidic valves of the type used to control flow of a fluid. More specifically, the present invention relates to pulse width modulated control of such fluid flow.

Fluidic valves have many applications and are generally used to control flow of a fluid between two locations. One simple valve configuration is a simple blocking element positioned in a pipe, or the like, which can be moved between at least two positions. In one position, fluid is allowed to flow through the pipe while in the other position, the blocking element seals or partially seals against the pipe and blocks or restricts flow of fluid. If multiple positions are available between the fully “on” position (with large opening) and the fully “off” position (completely closed), flow of fluid can be further controlled accordingly. Valves with adjustable partial openings are the most prevalent means of controlling the pressure or flow in a hydraulic circuit. However, flow through partially open valves induces pressure drops across the valve, and consequently throttling energy loss, given by the product of the pressure drop across the valve and the flow, is incurred. Thus, such throttling valves are inherently inefficient.

On the other hand, valves with binary positions—fully on or fully off, are inherently more efficient, since pressure drop is small when it is fully open, and flow is cut off when it is fully close. Thus, throttling loss in either positions can be zero or very small. In order to allow such on/off valves to achieve variable flow, the valve can be pulsed on and off at different times during the operation of the system. One such mode of operation is via pulse width modulation (PWM). In a pulse width modulated valve, the valve is rapidly switched between the fully on position and the fully off position. By changing the relative duration that the valve is in either the fully on position or the fully off position to the total period of an on/off cycle, the average flow rate can be accurately controlled between a maximum flow rate and zero flow rate. Such pulse width modulated valves can be used in many applications, for example, in achieving variable displacement functions from fixed displacement pumps and motors.

One example pulse width modulated valve configuration uses an obstruction which is moved linearly in a flow conduit between a fully blocking or closed position and a fully open position. The linear driving element can be, for example, an electromagnetic solenoid, a PZT actuator or the like. A critical factor in the performance of a pulse width modulated or other binary on/off valve configurations is the time it takes to transition between the fully on state, and the fully off state. Since the valve is throttling the flow during transition, it induces inefficiency. In a PWM valve, the proportion of time the valve is in transition relative to the time when it is fully on or fully off should be small to be efficient. On the other hand, a short cycling time (which consists of the fully on, fully off, and transition times) should be small for responsiveness and for precision. Thus, a short transition time is required for both efficiency as well as responsiveness and precision.

SUMMARY OF THE INVENTION

A pulse width modulated valve consists of an element which is in continuous unidirectional rotational motion. This element is driven by an external power source, or by the energy in the fluid flow. The motion of the rotating element is then translated to periodic high speed relative movement between a valve obstacle (land) and an inlet or exit port. By providing a means to modulate the relationship between the duration when the valve obstacle does or does not cover the inlet or exit port, the duty cycle of the PWM operation is modulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a mechanical hydraulic boost converter including a pulse width modulated fluidic valve in accordance with the present invention.

FIG. 2A is a perspective view of a pulse width modulated fluidic valve where the cylinder has been cut away through its axis to reveal the spool which travels in its bore in a first position.

FIG. 2B is a perspective view of a pulse width modulated fluidic valve where the cylinder has been cut away through its axis to reveal the spool which travels in its bore in a second position.

FIG. 3 is a perspective view of a rotatable spool shown in FIGS. 1 and 2.

FIG. 4A is a graph of flow versus time for the fluidic valve of FIGS. 2A and 2B in which the rotatable valve spool is in a first linear position.

FIG. 4B is a graph of flow versus time for the fluidic valve of FIGS. 2A and 2B in which the rotatable valve spool is in a second linear position.

FIG. 5 is a perspective view of another configuration of a rotatable spool.

FIG. 6 is a schematic diagram of another embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

One problem associated with the pulse width modulated valves described in the background section is that they must be positioned linearly at a relatively fast rate. Such linear positioning requires motion of the blocking element in one direction to be stopped, and the blocking element be accelerated rapidly in the opposite direction. This requires a large amount of force and energy, is difficult to control, and is stressful on the components of the valve. The force and power required to accelerate and decelerate the blocking element only are proportional to the second and third power of the velocity respectively. Additional force and power, proportional to first and second power of the velocity, are needed to overcome the friction. Thus, a large actuator and a significant amount of power are needed to achieve short transition times.

The present invention provides a pulse width modulated fluidic valve in which a unidirectional rotating element is used to generate high speed relative motion between a valve obstacle and an inlet/exit port. The invention further provides a means to modulate the relationship between the duration when the valve obstacle does or does not cover the inlet or exit port, thus modulating the pulse width. In the preferred embodiment, the rotating element is a rotatable spool which rotates within a cylinder. The rotatable spool provides a passage therethrough and the speed of rotation can be used to control the frequency of the fluidic pulses through the valve. Further, the configuration of the spool allows it to be moved axially relative to the cylinder such that the width of the pulse can be controlled.

-   -   When the obstacle covers the inlet/exit port, the valve is fully         off, when the obstacle does not cover the inlet/exit port, the         valve is fully on. Unlike a PWM valve that moves linearly         requiring starting and stopping, the unidirectional motion of         the proposed valve allows for the actuator to always tend to         accelerate the valve. Thus, the relative speed between the valve         obstacle and the port will be consistently high, achieving a         short transition time. A means to modulating the relationship         between the duration when the valve obstacle does or does not         cover the inlet or exit port is also provided. This serves to         modulate the duty cycle of the PWM operation. Various         embodiments can be developed based on this concept.

Our preferred embodiment of the proposed pulse width modulated fluidic valve includes a cylinder having an elongated bore. A first port and a second port extend from outside the cylinder into the bore. A rotatable spool is carried in the bore and is movable in a direction of the length of cylinder. The spool contains passages which allows fluid to flow between the non-blocking portion of the spool surface and the center bore of the spool. The spool has a variable blocking feature, which selectively blocks passage of fluid from the first and second ports to the center of the spool, as a function of angular position relative to the first and second ports and a function of linear position along the length of the cylinder. The rotatable spool is constantly rotating unidirectionally at high speed. This achieves a high relative speed between the spool and the inlet/exit port, achieving a short transition time. By translating the spool axially along the bore, the inlet/exit port will be exposed to varying blocking features, which can be designed to achieve variable duration when the valve is fully on or fully off.

FIG. 1 is a simplified diagram showing one application of a pulse width modulated fluidic valve in a mechanical-hydraulic boost converter. In this example, a continuously running pump 102 is driven by a motor 104 through a fly wheel 106. The pump 102 draws fluid from reservoir 110. A pulse width modulated fluidic valve 112 receives a control signal which controls the amount of fluid from pump 102 which is recirculated. The fluid which is not recirculated is pumped through a check valve 116 and an accumulator 114 is used to smooth out the pulses in the flow of the fluid. This provides a controllable flow of fluid to a hydraulic load, for example, a piston/cylinder arrangement. Thus, the fixed displacement pump 102 is allowed to achieve the function of a variable displacement pump.

FIGS. 2A and 2B are perspective views of a pulse width modulated fluid valve in accordance with one embodiment. The valve cylinder has been cut away through its axis to reveal the spool which travels in its bore. In FIG. 2A, the valve 112 is arranged in a mostly closed position. While in FIG. 2B, the valve 112 is arranged in a mostly open position. Valve 112 includes an elongate cylinder 130 having a bore 132 therein. A rotatable spool 134 is positioned within bore 132. Cylinder 130 also includes first and second ports 136 and 138 which extend from outside of the cylinder into the bore 132. Valve 112 also includes a rotary driver 144 and a linear driver 146 responsive to control signals 148 and 150, respectively. Drivers 144 and 146 couple to spool 134 through spool armature 152. Rotary driver 144 is configured to rotate spool 134 relative to ports 136 and 138 of cylinder 130 in response to control signal 148. Similarly, linear driver 146 is arranged to move spool 134 linearly within cylinder 130 along an axial length of the cylinder 130 in response to the control signal 150.

FIG. 3 is a more detailed perspective view of spool 134. As illustrated in FIG. 3, spool 134 includes variable blocking features 160, and end seals 162 and 164. These components are configured to fluidically seal the spool 134 with respect to the wall of bore 132 of cylinder 130. The variable blocking features 160 define a fluid blocking region 166 and a fluid flow region 168. In fluid flow region 168, passageways 170 extend through spool 134.

In the configuration of FIG. 3, variable blocking feature 160 is formed as a ridge in the outer circumference of spool 134 and comprises a first helical portion 160A and a second helical portion 160B.

Turning back to FIG. 2A, as spool 134 rotates, a fluidic passageway between ports 136 and 138 will be opened or closed depending upon the position of blocking features 160 relative to ports 136 and 138. Because of the linear position of spool 134 relative to portions 136 and 138, as the spool 134 rotates, the ports 136 and 138 will reside most of the time in the fluid flow blocking region 166 and flow of fluid will be blocked by portions 160A and 160B of blocking feature 160. However, as spool 134 continues to rotate, the ports 136 and 138 will less frequently reside within fluid flow region 168 such that there can be fluid flow between ports 136 and 138 through passageway 170.

FIG. 4A is a graph of flow versus time for this configuration. As shown in FIG. 4A, a series of relatively narrow flow pulses are provided with the valve being mostly off between each pulse. This provides a relatively small average flow level.

Returning to the configuration shown in FIG. 2B, the spool 134 is shown positioned further within cylinder 130. In this configuration, as spool 134 rotates, the ports 136 and 138 will reside for a greater period of time in the fluid flow region 168 of spool 134 than they will in the fluid blocking region 166. FIG. 4B is a graph of flow versus time for this arrangement. As illustrated in FIG. 4B, the flow comprises a series of relatively long flow periods with brief flow blocking periods in between each peak. This results in an average flow which is almost as great as the level of the individual peaks, and much greater than the average flow level shown in FIG. 4A. Thus, as illustrated above, the period of the pulses can be controlled by adjusting the rotation speed of rotary driver 144, while the width of the individual pulses can be controlled by adjusting the linear position of the spool 134 within the cylinder 130 using linear driver 146. Further, the relationship between linear position and pulse width can be controlled by changing the shape of the variable blocking features 160. As illustrated in FIG. 3, the variable blocking features 160 have a profile which is dependent upon both the angular position along the circumference of spool 134 as well as the linear position along the axis of spool 134.

FIG. 5 is a perspective view of another configuration of a spool 200. In the configuration of FIG. 5, the variable blocking feature 160 is formed as a step change in the outer surface of the spool 200. Such a configuration may be easier to manufacture and provide greater blocking abilities in comparison to that shown in FIG. 3. However, spool 134 shown in FIG. 3 provides less surface area against the wall of bore 132 and therefore should provide lower journal friction.

In general, a pulse width modulated (PWM) fluidic valve is provided. The valve can be cycled from on to off at high frequencies, for example, on the order of 1000 Hz. The flow through the valve is controlled by varying the fraction of each cycle that the valve is open. The flow rate through the valve is infinitely variable between zero flow and maximum flow. Despite its high frequency, the valve can also provide high fluid flow rates with low pressure drops. Pressure losses are minimized by providing sufficiently large port openings, and by reducing the time during which the switching port is partially obstructed by the valve spool. The spool of the valve is driven by a linkage having two degrees of freedom, one in a linear direction and one rotational. The valve is applicable to many types of installation, for example, a fixed displacement hydraulic pump in which the valve can control the output of flow of the pump; or a fixed displacement motor in which the valve can control the output speed of the motor at constant flow. Such a valve configuration is for use with hydraulic motors, hydraulic transformers, etc. This configuration provides a high frequency response which makes for superior operation as a pulse width modulated valve. The valve can be combined with a controller to provide software enabled features. For example, such software can be implemented in drivers 144 and 146, or in software which controls such drivers. The valve can operate at high frequencies which thereby improves controllability. The valve varies flow rate without throttling the flow which thereby reduces input power and lowers operating costs. Such a valve configuration provides for improved size, weight and efficiency over other configurations.

The above description of the present invention is for illustrative purposes only. The techniques and description set forth above may be modified as appropriate. For example, although only two ports are shown, other configurations could be used. For example, using additional ports will increase the fraction of each pulse cycle during which each port is partially obstructed by the blocking feature of the spool. In one configuration, three such ports may be desirable due to the stable nature of a triangular configuration. However, there is a trade off between additional ports and efficiency of the valve. The spool and cylindrical housing need merely be moved relative to one another. The actual movement, rotational or linear, can be by movement of any one of the spool or cylindrical housing or a combination of both. During operation, the angular velocity should exceed some minimum threshold for the valve to be operational. Once the minimal velocity has been met, the flow rate should be nominally independent of the angular velocity of the valve. Note that fluid inertia may start to effect the actual flow rate at high rates of pulsing. The rate of rotation of the spool sets the frequency of the pulses. In some configurations, the valve is coupled to an accumulator on the load side of the system, for example element 114 in FIG. 1. This enables averaging the discreet pulses of flow from the valve into a steady flow applied to the load with a “ripple” superimposed on top of the flow. Increasing the rate of pulsing reduces the amplitude of the ripple which is typically desirable. Further, increasing the angular velocity of the spool also increases the potential to control the bandwidth of the valve, i.e., the speed at which the valve can respond to a command to change the flow rate. Therefore, with the present invention, given flow rate the valve can pulse the flow at a higher rate than a linear valve. However, in general, the rate of rotation does not nominally change the average flow rate.

The particular actuator used to provide the relative rotation can be configured as desired. Although the Figures show an external motor configured to rotate the spool, other configurations can be used. For example, power can be extracted from the flow of fluid through the valve and used to rotate the spool. In other words, the spool serves as a fluid turbine as well as the means for starting and stopping the fluid flow. In such a configuration, the rotary actuator 144 as shown in the Figures is not required. Instead, ports through the cylinder sleeve, such as ports 136, 138 in FIG. 2A, may be configured tangentially to the circumference of the cylinder sleeve. In another example configuration, element 144 comprises a sensor which can be used to sense the rate of rotation of the spool as it is rotated by the fluid. This information can be used by a control algorithm.

In contrast to linear valves, in the present invention the fraction of the period that the fluid flow is partially blocked by the blocking feature traveling over the fluid ports in the sleeve is the same regardless of the frequency. In linear valves, the fraction of the cycle that the flow is partially blocked increases with frequency. The partially blocked state is undesirable in that the flow is choked and power is lost. Further, if the valve of the present invention is operated at high frequencies, the input power is not reduced. The power improvement results from achieving a variable flow without choking the flow through a variable orifice. In addition, as mentioned above, the valve can be run at high frequencies without increasing the relative small fraction of the cycle that the flow is choked.

Although the specific embodiments shown above illustrate one fluid path arrangement, of course, other arrangements can be used in accordance with the present invention. For example, the spool can be constructed to allow the flow of fluid out of the depression in the spool and in the axial direction. For example, referring to FIGS. 3 and 5, radial holes 170 may be removed. Alternatively, slots may be cut in the end seal 162 whereby fluid may flow out of the depression and along the axial direction. In yet another configuration, the seal 162 may be removed altogether. However, seal 162 may be advantageous in holding the spool concentrically with the sleeve. In another configuration, the valve can be constructed such that flow path is reversed, in other words, fluid can enter through the spool and exit through the ports in the sleeve. However, such a configuration may increase the volume of the fluid which is subjected to pulsing and thereby lowers the overall bandwidth of the valve system. In other words, a valve using the reverse flow path may not be able to provide the same performance of a valve using the forward path. However, such a valve may provide sufficient service at lower pulse frequencies. In yet another configuration, the center bore of the spool in FIG. 3 is divided into two separate chambers, one connected to spool feature 168 as before, and the other connected to feature 166 through new passages similar to ports 170. The two chambers are then connected to outlet ports. This configuration enables the valve to act as a three way valve that allows flow through either of the outlet ports.

The spool can be configured as desired. For example, the spool can be hollow in order to reduce mass. However, the spool may also be solid, or partially solid as desired. If a solid spool is used, some type of exit path should be provided for the fluid. This can be done in a number of different ways. In a first configuration, an axial escape path is provided for the fluid as discussed above. In another configuration, a hole is provided radially down into the spool with axial ports extending into the end of the spool to meet the radial holes. In yet another configuration, holes may be skewed between the radial and axial directions, i.e., to provide a single continuous hole which starts in the depression of the spool and exits at an axial face of the spool. This may also provide a rough technique for using fluid forces to cause the spool to rotate as discussed above. If fluid forces are used to spin the spool as discussed above, the rate of the rotation will not be constant nor directly controllable. However, as long as the fluid forces cause the spool to rotate above some minimum angular velocity, the valve will still be operational. The precise speed for proper valve operation is dependent upon spool configuration. However, it is preferable that the speed be maintained within some reasonable bounds.

A helical cut for the depression in the spool may be beneficial in that it implements a linear relationship between the axial position of the spool and the width of the “duty cycle” of each pulse. However, the depression may be cut with some alternative profiles to achieve the desired pulse profile. The invention is not limited in particular to a helical cut. Similarly, the ports in the cylindrical housing are not required to be positioned perfectly radially. In fact, in order to implement a spool which is driven by fluid flow forces, it may be desirable to skew these ports off of the radial direction. Skewing the center line of the port from the radial direction also has the negative consequence of increasing the fraction of each duty cycle that each port is partially obstructed by the blocking feature as discussed above. Therefore, a trade-off arises between the efficiency of pulsing the fluid flow and the efficiency of the fluid dynamics for directly spinning the valve.

Although the valve is described to be a pulse width modulated valve in that the duty ratio of the valve being fully on versus the cycle time is modulated, more precise control of the timing of when the valve is turned on and turned off can be attained using the invention. This can be achieved for example, in the configuration in FIG. 3, by moving the spool linearly so as to enable the ports 136/138 to avoid or to approach the blocking feature 160B in FIG. 3. This in turn lengthens or shortens the individual pulse width.

Although FIG. 1 shows one potential application for a valve of the present invention, the valve of the present application is applicable to any appropriate configuration. One embodiment is shown in FIG. 6 which consists of a rotating mechanism 250 with the obstacle block or valve spool 252 connected to arms 254,256 of the mechanism. A drive motor 258 coupled to ground 260 through link 262 drives the spool 252. Spool 252 moves in housing 264 and selectively blocks port 266. While a sliding obstacle block is suggested in FIG. 6, a rotating obstacle block could also be used. The modulating function can be achieved by sliding or rotating another link.

In general, the valve of the present invention allows pulsing of the flow of the fluid without requiring accelerating or decelerating of the valve spool. In the embodiment suggested in FIG. 6, while the valve spool does accelerate and decelerate, the rotating driving element does not require acceleration or deceleration. In some configurations, it is possible to vary the flow from zero flow to a maximum flow. However, the valve may also be configured such that the flow is only variable over some smaller fraction of the total possible range.

In one configuration, the spool is rotated continuously relative to the sleeve. In another configuration, the spool is rotated back and forth in the circumferential direction rather than continuously rotated.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although a cylindrical housing is shown the housing may be of any appropriate configuration. Similarly, although a particular spool configuration is illustrated, the spool can be of any appropriate shape. In another example configuration, a linkage or armature is connected radially offset from the spool and is used to rotate the spool using a reciprocating motion. In general, the present invention utilizes the continuous rotary motion of an element in order to achieve high frequency periodic motion which is used to move a valve obstacle. 

1. A pulse width modulated fluidic valve, comprising: a housing having an elongate bore, a length and first and second ports which extend from outside the housing into the bore; and a rotatable spool carried in the bore and movable in a direction of the length of housing, the spool having a variable blocking feature which selectively blocks passage of fluid between the first and second ports as a function of angular position relative to the first and second ports and as a function of linear position along the length of the housing.
 2. The apparatus of claim 1 including a rotary driver coupled to rotatable spool configure to rotate the rotatable spool.
 3. The apparatus of claim 2 wherein a speed of rotation is controllable.
 4. The apparatus of claim 2 wherein the speed of rotation is fixed.
 5. The apparatus of claim 1 includes a linear displacement driver coupled to the rotatable spool configured to move the spool in a direction along the length of the housing.
 6. The apparatus of claim 5 wherein the linear displacement driver is responsive to a linear position control input.
 7. The apparatus of claim 1 wherein the variable blocking feature comprises a seal which provides a seal between the spool and a wall of the bore.
 8. The apparatus of claim 1 wherein the seal extends linearly along a length of the spool and radially along a circumference of the spool.
 9. The apparatus of claim 8 wherein the seal is helical.
 10. The apparatus of claim 1 wherein the variable blocking feature comprises two seals which provide fluidic seals between an outer circumference of the rotatable spool and a wall of the elongate bore of the housing.
 11. The apparatus of claim 7 wherein the seal is formed relative to a cut out region of the spool.
 12. The apparatus of claim 7 wherein the seal comprises a raised portion on an outer circumference of the spool.
 13. The apparatus of claim 1 wherein the rotatable spool is hollow.
 14. The apparatus of claim 1 wherein the rotatable spool includes a fluidic passageway which extends radially through the spool.
 15. The apparatus of claim 14 wherein the passageway extends from one side of the variable blocking feature to another side of the variable blocking feature.
 16. A method of controlling flow of a fluid, comprising: providing flow of a fluid into a housing; receiving the flow of the fluid into the housing; rotating a spool within the housing, the spool including a variable blocking feature; moving the spool linearly within the housing; and receiving the fluid at an exit from the housing.
 17. The method of claim 16 including actuating a rotary driver coupled to rotatable spool configure to rotate the rotatable spool.
 18. The method of claim 17 wherein a speed of rotation is controllable.
 19. The method of claim 17 wherein the speed of rotation is fixed.
 20. The method of claim 16 includes actuating a linear displacement driver coupled to the rotatable spool configured to move the spool in a direction along the length of the housing.
 21. The method of claim 20 wherein the linear displacement driver is responsive to a linear position control input.
 22. The method of claim 16 wherein the variable blocking feature comprises a seal which provides a seal between the spool and a wall of the bore.
 23. The method of claim 22 wherein the seal extends linearly along a length of the spool and radially along a circumference of the spool.
 24. The method of claim 22 wherein the seal is helical.
 25. The method of claim 16 wherein the variable blocking feature comprises two seals which provide fluidic seals between an outer circumference of the rotatable spool and a wall of the elongate bore of the housing.
 26. The method of claim 22 wherein the seal is formed relative to a cut out region of the spool.
 27. The method of claim 22 wherein the seal comprises a raised portion on an outer circumference of the spool.
 28. The method of claim 16 wherein the rotatable spool is hollow.
 29. The method of claim 16 wherein the rotatable spool includes a fluidic passageway which extends radially through the spool.
 30. The method of claim 16 wherein the spool includes a passageway which extends from one side of the variable blocking feature to another side of the variable blocking feature. 